Currently lenses of various types are used to collimate a diverging light beam exiting an optical waveguide and to focus light that being launched into an optical waveguide so as to more efficiently couple the light.
One of the most ubiquitous building blocks used in the design and manufacture of optical elements is the graded index (GRIN) lens. Lenses of this type are produced under the trade name "SELFOC"; the mark is registered in Japan and owned by the Nippon Sheet and Glass Co. Ltd. GRIN lenses in combination with other optical elements are used in the manufacture of WDM devices, optical couplers, circulators, isolators, and other devices. The use of a GRIN lens in this invention provides a number of advantages over other conventional lenses, however does not limit the invention to only GRIN lenses.
Advantages of GRIN lenses are that they are relatively inexpensive, compact, and furthermore have parallel flat end faces. In particular, the flat end face of the GRIN lens allows a single lens to be used as a means of collimating or focusing the light, and as well, as a means of tapping light reflected from the end face of the lens.
Quarter pitch focusing/collimating GRIN lenses are known to be used having their collimating ends adjacent one another in a back to back relationship, and having a thin optical element such as a dichroic thin film filter sandwiched therebetween. Such an arrangement may serve as a multiplexing/dempultiplexing optical filter. One or more optical fibers are typically coupled to an input end and to an output end (outwardly facing ends of the lenses) of the device. For these filters to work efficiently, without high coupling losses, it is especially important for the dichoric element disposed between the two lenses to be very thin.
When two quarter pitch GRIN lenses are placed directly adjacent one another with their collimating ends coupled, light launched into the input end from an input optical fiber having its optical axis parallel to but offset with the optical axis of the GRIN lens is directed to a location at the output end of the second GRIN lens. The light leaving the second lens is directly coupled into an output fibre that is parallel with the optical axis and the input optical fibre but offset thereto. However, if the lenses are unduly spaced, light exiting the output (focusing) end exits at an angle to the optical axis of the lens and is difficult to couple to a waveguide, for example to an optical fibre that does not have its optical axis parallel with the optical axis of the lens. This is a particular problem when a relatively thick optical element such as an isolator is disposed adjacent to a GRIN lens.
This invention obviates this difficulty, by providing a lens arrangement that is compatible with a relatively thin or thick optical element disposed between input and output waveguides.
Polarization independent optical circulators generally comprise a birefringent optical element such a rutile crystal for splitting an incoming beam into two orthogonally polarized beams. These two oppositely oriented beams, are then individually operated upon by being passed through at least a non-reciprocal rotating element and at least a second beam shifting crystal that is oriented to shift a beam passing therethrough in a first direction, for example, from port 1 to port 2, and allowing a beam passing therethrough from port 2 to port 3 without shifting. Further, a birefringent crystal is provided for combining at, for example, port 2, the two beams that where originally separated according to polarization components, into a single beam.
The cost of providing a large birefringent crystal is usually significant. For example, a crystal having a dimension of 2 mm, as is suggested in some optical circulator designs, can, according to the teachings of this invention be cut into four crystals, thereby providing significant cost savings.
It is therefore an object of this invention, to provide an optical circulator that minimizes the size of costly optical components.